Optical storage media are media in which data are stored in an optically readable manner, for example by means of a laser and an optical detector, for example a photodetector, being integrated within a pickup. The detector is used for detecting reflected light of the laser beam when reading data on the storage medium. In the meanwhile a large variety of optical storage media are known, which are operated with different laser wavelength, and which have different sizes for providing storage capacities from below one Gigabyte up to 50 Gigabyte (GB). The formats include read-only formats such as Audio CD and Video DVD, write-once optical media such as CD-R and DVD-R, DVD+R, as well as rewritable formats like CD-RW, DVD-RW and DVD+RW. Digital data are stored on these media along tracks in one or more layers of the media.
The storage medium with the highest data capacity is at present the Blu-Ray disc (BD), which allows to store up to about 50 GB on a dual layer disc. For reading and writing of a Blu-Ray disc an optical pickup with a laser wavelength of 405 nm and a numerical aperture of 0,85 is used. On the Blu-Ray disc a track pitch of 320 nm and a mark length from 2 T to 8 T or 9 T is used, where T is the channel bit length and wherein 2 T corresponds with a minimum mark length of 138-160 nm.
New optical storage media with a super-resolution structure offer the possibility to increase the data density of the optical storage medium by a factor of two to four in one dimension as compared with the Blu-Ray disc. This is possible by including a nonlinear layer, which is placed above a data layer of the optical storage medium, and which significantly reduces the effective size of a light spot used for reading from or writing to the optical storage medium. The nonlinear layer can be understood as a mask layer because it is arranged above the data layer and for some specific materials only the high intensity center part of a laser beam can penetrate the mask layer. Further, semiconductor materials can be used as a nonlinear layer, e.g. InSb, which show a higher reflectivity in the center part of the focused laser beam, and for which the center reflectivity is dependent on the pit structure of the corresponding data layer.
Therefore, the super-resolution effect allows to record and read data stored in marks of an optical disc, which have a size below the diffraction limit of a corresponding optical pickup. The diffraction limit of a Blu-Ray type pickup having a laser wavelength λ=405 nm and a numerical aperture NA=0,85 is about λ/2NA=238 nm. The nonlinear layer is often called a super-resolution near-field structure (Super-RENS) layer because it is assumed that for some specific materials, the optical effect of reducing the effective spot size of the laser beam is based on a near-field interaction between the marks and spaces of the data layer and the nonlinear layer. A Super-RENS optical disc comprising a super resolution near-field structure formed of a metal oxide, a polymer compound or a phase change layer comprising a GeSbTe or a AgInSbTe structure for recording of data and reproducing of data is known from WO 2005/081242 and U.S. 2004/0257968. Further examples of super-resolution optical media are described in WO 2004/032123.